The present invention relates to a dielectric filter to be used in a high-frequency range.
Dielectric filters generally compose a plurality of resonators, the interstage coupling being made using coils (for example, air-core coils). This provision of the coils requires an installation space whereby difficulty is encountered to achieve the size-reduction, particularly reduction of the dimension of the dielectric filter in the height directions. In addition, since the coil are required to be mounted on a substrate, the provision thereof results in being troublesome. Still further, since the inductances of the coils considerably scatters, the inductance adjustments for the coils are required after assembling. One possible solution is to pattern the coils on a substrate. However, one problem faced in patterning the coils is to greatly increase insertion loss of the filter because the Q factor of the patterned coil is limited to about 10 (900 MHz).
Prior to describing an embodiment of the present invention, a brief description of conventional dielectric filers will be described hereinbelow for a better understanding of the present invention. FIGS. 1A and 1B are illustrations of a conventional dielectric filter, FIG. 1A being a perspective view showing the conventional dielectric filter and FIG. 1B being a cross-sectional view taken along a line B--B. In FIGS. 1A and 1B, the conventional dielectric filter is composed of quarter-wave coaxial type resonators 101 to 104 each of which may be made such that BaTi.sub.4 O.sub.9 based ceramic powder is produced and then baked so as to form conductive layers at the inside and outside thereof before removing the conductive layers presented at the upper portion of an outer conductor. Each of the conductive layers is formed by means of the printing of an Ag paste or the like. It is also appropriate to form the conductive layer by means of the Cu plating. Illustrated at numeral 105 is a dielectric substrate which is an Al.sub.2 O.sub.3 ceramic substrate, Ba.sub.2 Ti.sub.9 O.sub.20 based ceramic substrate, BaO--TiO.sub.2 --Sm.sub.2 O.sub. 3 based ceramic substrate or the like. The dielectric substrate 105 has an arrangement as illustrated in FIGS. 1C to 1E. FIG. 1C is a top surface illustration of the dielectric substrate 105, FIG. 1D is a side illustration thereof and FIG. 1E is a bottom surface illustration thereof. In FIGS. 1C to 1E, on the front surface of the dielectric substrate 105 there are provided conductors 107, 108, 118 and 120, and on the bottom surface thereof there are provided conductors 116, 117, 128 and 129. Here, numerals 113, 114 and 115 respectively represent air-core coils. Further, In FIGS. 1A and 1B, numerals 121 and 122 respectively designate input and output terminals, 123 depicts a housing and 124 to 127 are respectively central conductors. In the dielectric filter thus arranged, capacitances are formed between the conductors 118 and 128 and further between the conductors 120 and 129 and the interstage couplings are made through the air-core coils 113 to 115. Further, for polarizing, in ranges between the input and output terminals 121, 122 and the interstage resonators 102, 103 contacting with the input/output side resonators 101, 104, capacitances are constituted between the conductors 107 and 116 constructed on the dielectric substrate 105 and further between the conductors 108 and 117 similarly constructed thereon, thereby forming a plurality of poles as illustrated in FIG. 1G. FIG. 1F shows an equivalent circuit of the conventional dielectric filter. As described above, the conventional dielectric filter has disadvantages in that difficulty is encountered to achieve the size-reduction and further to allow the mass production.